INDEX

FIELD OF THE INVENTION

The present invention relates to a new relief precursor for flexographic printing elements, and the method for making such. The relief precursor is exposed to electromagnetic radiation in an imaging manner whereby exposed parts of a photosensitive layer change their solubility or melting behavior. Such flexographic printing elements are widely used in printing surfaces.

BACKGROUND OF THE INVENTION

Flexographic printing elements are well known in the art and are especially useful for commercial printing on diverse products such as flexible plastic containers, cartons, plastic bags, boxes and envelopes. For the purpose of this specification, uncured plates to be used for preparing (cured) flexographic printing elements are referred to as relief precursors. Relief precursors typically comprise a layer prepared from a photo-curable polymer composition on the side that is to be used for printing, which may be selectively cured by exposing the photo-curable layer image- wise to light, e.g. UV light. The unexposed (uncured) parts of the layer may then be removed in developer baths, typically with an organic solvent or aqueous solutions. After drying and optional post exposure, the flexographic printing element is ready for use. It will be appreciated that the removal of uncured parts (developing) of the flexographic printing elements must be done in a precise manner. Any unintentional uncured residue that is left on the flexographic printing plate may lead to an unclear image on the flexographic printing plate, and, hence unclear prints.

Another way to prepare printing elements from a relief precursor is to expose the relief precursor to electromagnetic radiation in an imaging manner whereby exposed parts of a photosensitive layer change their solubility or melting behavior. The difference in melting or solubility allows selective removal of unexposed material to form a relief printing plate, which is then used to transfer ink from the printing plate to a printing substrate. Removal of unexposed material may be achieved by treating the precursor with a developing liquid, which dissolves unexposed material, or by a thermal treatment, which liquefies the unexposed material.

In EP-A-0332070 a method is described wherein unexposed material is dissolved in water, aqueous solutions or solvents and solvent mixtures, in combination with mechanical interaction by brushes in a so-called developing unit. Another option is to remove liquefied material by continuously contacting it with an absorbing material. The absorbing developer material may be a non-woven of polyamide, polyester, cellulose or inorganic fibers onto which the softened material is adhering and subsequently removed. Such methods are described for example in US-A-3264103, US-A-5175072 or WO-A-9614603.

Even though the technology is on the market for quite a while, there are still some problems to be solved or properties to be improved. A disadvantage of the above described flexographic printing element precursors is that the torque and energy consumption in the production process requires some improvement. Another disadvantage is that in the production device of the flexographic printing precursor the temperature can still be improved.

Accordingly, there is a demand for flexographic printing element precursors that can be produced with an improved torque. There is furthermore a demand for a more time and energy efficient production.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a relief precursor that has a high melt flow index (MFI) and at the same time, the resulting relief plates have a fine-tuned hardness comparable to relief plates with standard plasticizers. An increased MFI allows producing the relief precursors with lower torque in the production device. In addition, the temperature in the production device may be lowered and therefore time and energy are saved. It is another object of the invention to use a process with a lower torque in order to prolong the lifetime of the equipment. It is yet another object of the present invention to develop a practical method for producing a printing plate precursor that will save energy and reduce energy consumption.

Accordingly, the present invention relates to a relief precursor as claimed in claim 1. In particular, a relief precursor is provided comprising a dimensionally stable support, and at least one photopolymer layer comprising at least one binder, at least one photoinitiator or photoinitiating system, at least one component with at least one unsaturated group, at least one plasticizer, wherein the melt flow index (MFI) of a mixture of 30 wt% of the at least one plasticizer with a SIS block copolymer (with a styrene content of 19%, a diblock content of 30%, and a molecular weight Mw 216000 g/mol, a dispersity of 1.04) is higher than 7 g/10 min (MFI 160 °C/1.2 kg). By using a relief precursor comprising a plasticizer characterized by a melt flow index (MFI) of a mixture of 30 wt% of the at least one plasticizer with a SIS block copolymer (with a styrene content of 19%, a diblock content of 30%, and a molecular weight Mw 216000 g/mol, a dispersity of 1.04) higher than 7 g/10 min (MFI 160 °C/1.2 kg), the resulting relief plates have a fine-tuned hardness comparable to relief plates with standard plasticizers. Furthermore, an increased MFI allows to produce the relief precursors with a lower torque in the production device. In addition, the temperature in the production device may be lowered and therefore time and energy are saved. Furthermore, deeper reliefs on (thermally developable) relief plates are being generated, resulting in more consistent printing results. Furthermore, development times are shorter, and energy savings due to reduced temperature and/or development time are achieved.

The present invention also relates to a method for generating a relief structure comprising the steps of: a.) the provision of a relief precursor comprising i) a dimensionally stable support, ii) at least one photopolymer layer comprising at least one binder, at least one photoinitiator or photoinitiating system, at least one component with at least one unsaturated group and at least one plasticizer, wherein the melt flow index of a mixture of 30 wt% of the at least one plasticizer with a SIS block copolymer (with a styrene content of 19%, a diblock content of 30%, and a molecular weight Mw 216000 g/mol, a dispersity of 1.04) is higher than 7 g/10 min (MFI 160 °C/1.2 kg), b.) imaging the relief precursor, c.) exposing the imaged relief precursor with electromagnetic radiation to cure the imaged areas, d.) removing of the non-cured areas, and e.) optionally further steps.

DETAILED DESCRIPTION OF THE INVENTION

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art, which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

Thus, in the present invention a relief precursor is provided comprising a dimensionally stable support, at least one photopolymer layer comprising at least one binder, at least one photoinitiator or photoinitiating system, at least one component with at least one unsaturated group and at least one plasticizer, wherein the melt flow index (MFI) of a mixture of 30 wt% of the at least one plasticizer with a SIS block copolymer (with a styrene content of 19%, a diblock content of 30%, and a molecular weight Mw 216000 g/mol, a dispersity of 1.04) is higher than 7 g/10 min (MFI 160 °C/1.2 kg). One of the advantages of this invention is that the resulting relief plates have a fine-tuned hardness comparable to relief plates with standard plasticizers.

In the present invention a relief precursor is provided comprising a dimensionally stable support, at least one photopolymer layer comprising at least one binder, at least one photoinitiator or photoinitiating system, at least one component with at least one unsaturated group and at least one plasticizer wherein the MFI of a mixture of 30 wt% of the at least one plasticizer with an SIS block copolymer (styrene content 19%, diblock content 30%, molecular weight Mw 216000 g/mol, dispersity 1.04) is higher than 200% compared to the MFI with mineral oil (kinematic viscosity at 40 °C of 70 mm ² /s), preferably higher than 350%.

The SIS rubber is a styrene-isoprene-styrene rubber with a styrene content of 19%, a diblock content of 30% and a molecular weight of the triblock fraction of M _w =216000 g/mol, dispersity 1.04. The molecular weight of the diblock fraction is half of the molecular weight of the triblock.

Another rubber that is used is an SBS rubber. The SBS rubber is a styrene-butadiene- styrene rubber with a styrene content of 26.9 - 29.9% and a diblock content of 15% and a molecular weight of the triblock fraction of M _w =104000 g/mol, dispersity 1.02. The molecular weight of the diblock fraction is half of the molecular weight of the triblock.

Thus, in the present invention a relief precursor is provided comprising a dimensionally stable support, at least one photopolymer layer comprising at least one binder, at least one photoinitiator or photoinitiating system, at least one component with at least one unsaturated group and at least one plasticizer, wherein the melt flow index (MFI) of a mixture of 30 wt% of the at least one plasticizer with a SBS rubber with a styrene content of 26.9 - 29.9% and a diblock content of 15% and a molecular weight of the triblock fraction of M _w = 104000 g/mol, dispersity 1.02 is higher than 9 g/10 min (MFI 160 °C/1.2 kg). Preferably the MFI with the SBS rubber is higher than lOg/10 min (MFI 160 °C/1.2 kg) and more preferably higher than 20g/10 min (MFI 160 °C/1.2 kg).

A relief precursor to be used with the claimed processes is described in the following: A relief precursor generally comprises a dimensionally stable support or a supporting layer made of a first material and an additional layer made of a second material, which is different from said first material. The dimensionally stable support may be a flexible metal, a natural or artificial polymer, paper or combinations thereof. Preferably, the dimensionally stable support is a flexible metal or polymer film or sheet. In case of a flexible metal, the supporting layer could comprise a thin film, a sieve like structure, a mesh like structure, a woven or non-woven structure or a combination thereof. Steel, copper, nickel or aluminum sheets are preferred and may be about 50 to 1000 pm thick. In case of a polymer film, the film is dimensionally stable but bendable and may be made for example from polyalkylenes, polyesters, polyethylene terephthalate, polybutylene terephthalate, polyamides und polycarbonates, polymers reinforced with woven, non-woven or layered fibers (e.g. glass fibers, carbon fibers, polymer fibers) or combinations thereof. Preferably polyethylene and polyester foils are used and their thickness may be in the range of about 100 to 300 pm, preferably in the range of 100 to 200 pm.

The relief precursor preferably carries at least one further layer. For example, the further layer may be any one of the following: a direct engravable layer (e.g. by laser), a solvent or water developable layer, a thermally developable layer, a photosensitive layer, a combination of a photosensitive layer and a mask layer. More preferably, the further layer is an adhesion layer below the support, an adhesion layer between the support and a photopolymer layer or between any other layers, a barrier layer, a laser ablatable layer and/or a protective layer, or combinations thereof. Optionally there may be provided one or more further additional layers on top of further layer. Such one or more further additional layers may comprise a cover layer at the top of all other layers, which is removed before the imageable layer is imaged. The one or more further additional layers may comprise a relief layer, and an anti-halation layer between the supporting layer and the relief layer or at a side of the supporting layer, which is opposite of the relief layer. The one or more further additional layers may comprise a relief layer, an imageable layer, and one or more barrier layers between the relief layer and the imageable layer, which prevent diffusion of oxygen. Between the different layers described above one or more adhesion layers may be located, which ensure proper adhesion of the different layers.

The relief precursor comprises at least a photopolymer layer and may further comprise a mask layer. The mask layer may be ablated or changed in transparency during the treatment and forms a mask with transparent and non-transparent areas. Underneath of transparent areas of the mask the photosensitive layer undergoes a change in solubility and/or fluidity upon irradiation. The change is used to generate the relief by removing parts of the photosensitive layer in one or more subsequent steps. The change in solubility and/or fluidity may be achieved by photo-induced polymerization and/or crosslinking, rendering the irradiated areas less soluble and less meltable. In other cases the electromagnetic radiation may cause breaking of bonds or cleavage of protective groups rendering the irradiated areas more soluble and/or meltable. Preferably a process using photo-induced crosslinking and/or polymerization is used.

The relief precursor comprises a photopolymer layer comprising at least one photoinitiator or photoinitiating system. A photo-initiator is a compound, which upon irradiation with electromagnetic radiation may form a reactive species, which can start a polymerization reaction, a crosslinking reaction, a chain or bond scission reaction, which leads to a change of the solubility and/or meltability of the composition. Photo-initiators are known, which cleave and generate radicals, acids or bases. Such initiators are known to the person skilled in the art and described e.g. in: Bruce M. Monroe et ah, Chemical Review, 93, 435 (1993), R. S. Davidson, Journal of Photochemistry and Biology A: Chemistry, 73, 81 (1993) M. Tsunooka et ah, 25 Prog. Polym. Sci., 21, 1 (1996), F. D. Saeva, Topics in Current Chemistry, 1 56, 59 (1990), G. G. Maslak, Topics in Current Chemistry, 168, 1 (1993), H. B. Shuster et ah, JAGS, 112, 6329 (1990) and I. D. F. Eaton et ah, JAGS, 102, 3298 (1980), P. Fouassier and J. F. Rabek, Radiation Curing in Polymer Science and Technology, pages 77 to 117 (1993) or K.K. Dietliker, Photoinitiators for free Radical and Cationic Polymerisation, Chemistry & Technology of UV & EB Formulation for Coatings, Inks and Paints, Volume, 3, Sita Technology LTD, London 1991; or R.S. Davidson, Exploring the Science, technology and Applications of U.V. and E.B. Curing, Sita Technology LTD, London 1999. Further initiators are described in JP45-37377, JP44-86516, US3567453, US4343891, EP109772, EP109773, JP63138345, JP63142345, JP63142346, JP63143537, JP4642363, JP59152396, JP61151197, JP6341484, JP2249 and JP24705, JP626223, JPB6314340,

JP 1559174831, JP 1304453 und JP1152109.

The photopolymer layer of the relief precursor comprises at least one binder. The binders according to the invention are linear, branched or dendritic polymers, which may be homopolymers or copolymers. Copolymers can be random, alternating or block copolymers. As binder, those polymers, which are either soluble, dispersible or emulsifiable in either aqueous solutions, organic solvents or combinations of both are used. Suitable polymeric binders are those conventionally used for the production of letterpress printing plates, such as completely or partially hydrolyzed polyvinyl esters, for example partially hydrolyzed polyvinyl acetates, polyvinyl alcohol derivatives, e. g. partially hydrolyzed vinyl acetate/alkylene oxide graft copolymers, or polyvinyl alcohols subsequently acrylated by a polymer-analogous reaction, as described, for example, in EP-A-0079514, EP-A-0224164 or EP-A-0059988, and mixtures thereof. Also suitable as polymeric binders are polyurethanes or polyamides, which are soluble in water or water/ alcohol mixtures, as described, for example, in EP-A-00856472 or DE- A- 1522444. For flexographic printing precursors elastomeric binders are used. The thermoplastic-elastomeric block copolymers comprise at least one block, which consists essentially of alkenylaromatics, and at least one block, which consists essentially of 1,3-dienes. The alkenylaromatics may be, for example, styrene, a- methylstyrene, or vinyltoluene. Styrene is preferable. The 1,3-dienes are preferably butadiene and/or isoprene. These block copolymers may be linear, branched, or radial block copolymers. Generally speaking, they are triblock copolymers of the A-B-A type, but they may also be diblock polymers of the A-B type, or may be polymers having a plurality of alternating elastomeric and thermoplastic blocks. A-B-A-B-A, for example. Mixtures of two or more different block copolymers may also be used. Commercial triblock copolymers frequently include certain fractions of diblock copolymers. The diene units may be 1,2- or 1,4-linked. Also possible for use, furthermore, are thermoplastic elastomeric block copolymers with styrene and blocks and a random styrene-butadiene middle block. Use may also be made, of course, of mixtures of two or more thermoplastic-elastomeric binders, provided that the properties of the relief-forming layer are not negatively impacted as a result. As well as the stated thermoplastic-elastomeric block copolymers, the photopolymerizable layer may also comprise further elastomeric binders other than the block copolymers. With additional binders of this kind, also called secondary binders, the properties of the photopolymerizable layer can be modified. Examples of a secondary binder are vinyltoluene-a-methylstyrene copolymers. These polymer binders account for in general from 20 to 98%, preferably from 50 to 90% by weight of the total amount of the layer.

The photopolymer layer comprises furthermore at least one component with at least one unsaturated group. Preferably, these components are reactive compounds or monomers, which are suitable for the preparation of the mixtures are those, which are polymerizable and are compatible with the binders. Useful monomers of this type generally have a boiling point above 100 °C. They usually have a molecular weight of less than 3000 g/mol, preferably less than 2000 g/mol. More preferably, the ethylenically unsaturated monomers are used that ought to be compatible with the binders, and they have at least one polymerizable, ethylenically unsaturated group. As monomer it is possible in particular to use esters or amides of acrylic acid or methacrylic acid with mono- or polyfunctional alcohols, amines, aminoalcohols or hydroxyethers and hydroxyesters, esters of fumaric acid or maleic acid, and allyl compounds. Esters of acrylic acid or methacrylic acid are even more preferred. Preference is given to 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, 1,9-nonanediol diacrylate, or trimethylolpropane tri(meth)acrylate. Mixtures of different monomers can of course be used. The total amount of all the monomers used in the relief-forming layer together is generally 1 to 20 wt%, preferably 5 to 20 wt%, based in each case on the sum of all the constituents of the relief-forming layer. The amount of monomers having two ethylenically unsaturated groups is preferably 5 to 20 wt%, based on the sum of all constituents of the relief-forming layer, more preferably 8 to 18 wt%.

The photopolymer layer may comprise further components. The further components are selected from the group consisting of a further polymer, a filler, a plasticizer, an anti-blocking agent, a monomer, an additive (e.g. a stabilizer, a dye), a stabilizer, a crosslinker, a binder, a color forming compound, a dye, a pigment, an antioxidant and combinations thereof.

The relief precursor comprises a photopolymer layer as described above and may furthermore comprise a mask layer, the mask layer comprising at least a compound capable of absorbing electromagnetic radiation and a component capable of being removed by ablation (also known as digital plate precursor). Preferably, the mask layer is an integral layer of the relief precursor and is in direct contact with the photosensitive layer or with a functional layer disposed between photosensitive layer and mask layer. This functional layer is preferably a barrier layer and blocks oxygen. The mask layer may be imageable by ablation and removable by solvents or by thermal development. The mask layer is heated and removed by irradiation with high energy electromagnetic radiation, whereby an image wise stmctured mask is formed, which is used to transfer the structure onto the relief precursor. In order to do so the mask layer may be non transparent in the UV region and absorb radiation in the VIS-IR region of the electromagnetic spectrum. The VIS-IR radiation may then be used to heat and ablate the layer. The optical density of the mask layer in the UV region between 330 and 420 nm is in the range of 1 to 5, preferably in the range of 1.5 to 4 and more preferably in the range of 2 to 4.

The layer thickness of the ablatable mask layer may be in the range of 0.1 to 5 pm, preferably 0.3 to 4 pm, more preferably 1 to 3 pm. The laser sensitivity of the mask layer (measured as energy needed to ablate 1 cm ² ) may be in the range of 0.1 to 10 J/cm ² , preferably in the range of 0.3 to 5 J/cm ² , most preferably in the range of 0.5 to 5 J/cm ² .

The photopolymer layer comprises at least one plasticizer, wherein the at least one plasticizer is a bio-based plasticizer. Bio-based plasticizers are at least partially derived from renewable, biological resources such as plants (e.g. agricultural crops or wood), microorganisms (e.g. algae or yeasts), or animals. Bio-based plasticizers are environmental friendly, often but not necessarily biodegradable. Bio-based materials may be chemically altered, or modified with synthetic compounds, to change for example physical and or chemical properties. They then still remain bio-based materials. These plasticizers are generally used to maintain softness and flexibility at varying temperature ranges. These plasticizers can at least partly replace other plasticizers, like synthetic plasticizers. Many of the non-bio-based plasticizers, especially the so- called phthalates, are harmful to one's health and may affect the hormone balance. Others are mineral oil or polybutadiene based and not easily biodegradable. It is thus advantageous to at least partly replace them by bio-based plasticizers. We furthermore found that when bio-based plasticizers are being used for thermal development, a higher relief depth can be achieved for all concentrations investigated. Moreover, we found that the use of for example rapeseed oil instead of mineral oil allows for higher washout speeds and results in shorter process times. Preferably, the bio-based plasticizer is a vegetable oil, a fatty acid and/or a fatty acid ester of mono- or polyfunctional alcohols. More preferably, the bio-based plasticizer is one or more of rapeseed oil, sunflower oil, soybean oil, palm oil, palm kernel oil, coconut oil, medium-chain triglycerides (MCT) oil and or linseed oil. Further examples are aqai oil, adeps lanae, ahiflower oil, algea oil, aloe vera, amaranth oil, apricot kernel oil, argan oil, avocado oil, babassu oil, baobab oil, beeswax, black cumin oil, black cumin seed oil, blackcurrant oil, borage oil, brazilnut oil, broccoliseed oil, calendula oil, camelina oil, candelilla wax, carnauba wax, castor oil, chia oil, Chilean hazelnut oil, cocoa butter, corn oil, cotton seed oil, cupuacu butter, evening primrose oil, fish oil, glycerol, grape seed oil, groundnut oil, hazelnut oil, hemp oil, high-oleic canola oil, high oleic soybean oil, high oleic sunflower oil, illipe butter, jatropha curcas oil, jojoba oil, kukuinut oil, lanolin, laurel oil, macadamia nut oil, mango butter, manketti oil, marula oil, meadowfoam seed oil, milk thistle oil, moringa oil, murumuru butter, mustardseed oil, olive oil, olus oil, omega-3-6-9-oil, palmolein, palm stearin, paradise nut oil, passionfmit seed oil, peach kernel oil, peanut oil, pecan nut oil, perilla oil, pistachio nut oil, plum kernel oil, pomegranate oil, poppyseed oil, pumpkin seed oil, rapeseed oil hydrogenated, raw wool grease, rice bran oil, rose hip kernel oil, sacha inchi oil, safflower oil, sal fat, sea buckthorn oil, sesame oil, shea butter, soybean oil hydrogenated, soybean oil partially hydrogenated, squalane, sunflower oil hydrogenated, sunflower wax, sumac wax, tallow, tamanu oil, walnut oil, wheat germ oil, wool fat, wool alcohols.

The composition and properties of vegetable oils and fats are highly dependent on many factors. The fatty acid composition and impurities in the oil influence chemical and physical properties such as the UV transmission, light transmission, Gardner Color and iodine value. Main influence factors are: crop type, growing region, breed, refining and processing (such as filtering, bleaching, neutralizing, deodorizing). Moreover, oil compositions and properties can be modified by blending, distillation, fractionation, hydrogenation, interesterification with chemical catalysts, interesterification with specific lipases, enzymatic enhancement, biological solutions, domestication of wild crops, conventional seed breeding, (intra-species) genetic engineering, lipids from micro-organisms or other unconventional sources. Depending these factors, the chemical composition and properties of vegetable oils from different crop types can be more similar to one another than within one type of crop.

Within the same batch of oil, the properties may be dependent on the storage. A long storage time, a high storage temperature and/or contact with air and or oxygen may lead to aging of the oil. Possible effects may be e.g. a decrease in the UV transmission, light transmission and iodine value or an increase in Gardner Color and hydroxyl value. Fresh and well-processed oils with a high UV transmission and/or high light transmission and/or low Gardner Color and/or low hydroxyl value are preferred compared to aged oils or crude vegetable oils with a low UV transmission and or low light transmission and/or high Gardner Color and/or high hydroxyl value. Slow aging and storage over time can be accelerated by heating the oil under the influence of air.

The photopolymer layer comprises at least one plasticizer, wherein the at least one plasticizer is preferably a bio-based plasticizer. More plasticizers might also be present, it can thus be a mixture of 2 or more bio-based plasticizer, or a mixture of at least a bio-based plasticizer and conventional plasticizer(s). Thus, mixtures of different plasticizers may also be used, as long as at least one plasticizer is preferably a bio-based plasticizer. Examples of the other suitable plasticizers encompass modified and unmodified natural oils and natural resins, such as high- boiling paraffinic, naphthenic or aromatic mineral oils, synthetic oligomers or resins such as oligostyrene, high-boiling esters, oligomeric styrene-butadiene copolymers, oligomeric alpha-methylstyrene/ p-methylstyrene copolymers, liquid oligobutadienes, especially those having a molecular weight of 500 to 5000 g/mol, or liquid oligomeric acrylonitrile -butadiene copolymers or oligomeric ethylene - propyl-ene-diene copolymers. Preference is given to polybutadiene oils (liquid oligobutadienes), especially those having a molecular weight of 500 to 5000 g/mol, high-boiling aliphatic esters such as, in particular, alkyl esters of monocarboxylic and dicarboxylic acids, examples being stearates or adipates and mineral oils. Particularly preferred are high-boiling, substantially paraffinic and or naphthenic mineral oils. It is possible, for example, to use what are called paraffin-base solvates and specialty oils. With mineral oils, the skilled person distinguishes between technical white oils, which may also include a very small aromatic content, and medical white oils, which are substantially free from aromatics. They are commercially available and equally well-suited. Particularly widespread as plasticizers are white oils or oligomeric plasticizers, such as, in particular, polybutadiene oils, carboxylic esters, phthalates. In this regard, reference may be made by way of example to EP 992 849 and EP 2279454. The amount of a plasticizer optionally present is determined by the skilled person according to the desired properties of the layer. The Melt Flow Index (MFI) is a measure of the ease of flow of the melt of a thermoplastic polymer. It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied via prescribed alternative gravimetric weights for alternative prescribed temperatures. Polymer processors usually correlate the value of MFI with the polymer grade that they have to choose for different processes, and most often this value is not accompanied by the units, because it is taken for granted to be g /10 min. Similarly, the test conditions of MFI measurement is normally expressed in kilograms rather than any other unit. The method is described in the similar standards ASTM D1238. According to the invention, the melt flow index (MFI) of a mixture of 30 wt% of the at least one plasticizer with a SIS block copolymer (with a styrene content of 19%, a diblock content of 30%, and a molecular weight Mw 216000 g/mol, a dispersity of 1.04) is higher than 7 g/10 min (MFI 160 °C/1.2 kg). Preferably, the MFI of a mixture of 30 wt% of the at least one plasticizer with a SIS block copolymer (with a styrene content of 19%, a diblock content of 30%, and a molecular weight Mw 216000 g/mol, a dispersity of 1.04) is higher than 10 g/10 min, more preferably higher than 15 g/10 min (MFI 160 °C/1.2 kg).

When bio-based plasticizers are added to SBS based photosensitive formulations the melt flow index is increased also. For example the melt flow index of Kraton 1102 EU with a plasticizer concentration of 30% is increased to values higher than 9 g/10 min (MFI 160 °C/1.2 kg) or to values higher than 20 g/10 min (MFI 160 °C/1.2 kg).

Preferably, the plasticizer is characterized by a UV transmission at 365 nm of a solution of 5 wt% plasticizer in n-hexane of higher than 15%, more preferably higher than 30%, even more preferably higher than 50%, most preferably higher than 60%.

Advantageously, the plasticizer is characterized by a light transmission in comparison to medium-chain triglycerides (MCT)-oil (= 100% transmission) of higher than 78%. The measurement of the transparent MCT-oil, which is more resistant towards oxidation due to the lack of double bonds, was set as standard with 100% light transmission.

The plasticizer might be characterized by its Gardner Color. The Gardner Color Scale is a one-dimensional scale used to measure the shade of the color yellow. The Gardner scale and the APHA/Pt-Co/Hazen Color Scale overlap with the Gardner scale measuring higher concentrations of yellow color and the APHA scale measuring very low levels of yellow color. Colors of transparent liquids have been studied visually since the early 19th century. Changes in color can indicate contamination or impurities in the raw materials, process variations, or degradation of products over time. Advantageously, the plasticizer is characterized by a Gardner color lower than 7 according to ISO 4630:2015.

The plasticizer can also be characterized by its hydroxyl value. The hydroxyl value is defined as the number of milligrams of potassium hydroxide required to neutralize the acetic acid taken up on acetylation of one gram of a chemical substance that contains free hydroxyl groups. The hydroxyl value is a measure of the content of free hydroxyl groups in a chemical substance, usually expressed in units of the mass of potassium hydroxide (KOH) in milligrams equivalent to the hydroxyl content of one gram of the chemical substance. The analytical method used to determine the hydroxyl value traditionally involves acetylation of the free hydroxyl groups of the substance with acetic anhydride in pyridine solvent. After completion of the reaction, water is added, and the remaining unreacted acetic anhydride is converted to acetic acid and measured by titration with potassium hydroxide. The hydroxyl value can then be calculated. Advantageously, the plasticizer is characterized by a hydroxyl value according to ASTM D 1957-86 below 430, preferably below 250, even more preferably below 168.

The plasticizer can similarly be characterized by its iodine value. The iodine value (or iodine adsorption value or iodine number or iodine index, commonly abbreviated as IV) in chemistry is the mass of iodine in grams that is consumed by 100 grams of a chemical substance. Iodine numbers are often used to determine the degree of unsaturation in fats, oils and waxes. In fatty acids, unsaturation occurs mainly as double bonds, which are very reactive towards halogens, the iodine in this case. Thus, the higher the iodine value, the more unsaturated bonds contains the fat. The plasticizer to be used in the relief precursor of the invention preferably has an iodine value according to ISO 3961:2018 below 200, more preferably below 150.

The plasticizer can furthermore be characterized by its Hansen solubility parameter 5t. The Hansen solubility is based on the idea that like dissolves like where one molecule is defined as being 'like' another if it bonds to itself in a similar way. A description of the determination of the Hansen solubility parameters can be found in J. Brandrup, E.H. Immergut, E. A. Grulke, Polymer Handbook 4th ed., Wiley, New York, 1999, pp. VII / 675 - VII / 714. Advantageously, the plasticizer has a Hansen solubility parameter 5t in the range of from 16.0 up to 20.0, more preferably in the range of from 16 up to 17.5.

The amount of a plasticizer optionally present is determined by the skilled person according to the desired properties of the layer. The concentration of the plasticizer in the photopolymer layer is preferably in the range of from 3 up to 70 wt%, more preferably in the range of from 5 up to 65 wt%, even more preferably in the range of from 10 up to 65 wt%, most preferably in the range of from 20 up to 60 wt%, based on the total weight of the photopolymer layer. A thermal treatment may be utilized, for example, to initiate and/or to complete reactions, to increase the mechanical and or thermal stability of the relief structure, and to remove volatile constituents. For the thermal treatment, it is possible to use the known techniques, such as heating using heated gases or liquids, IR radiation, and any desired combinations thereof, for example. In these contexts, it is possible to employ ovens, blowers, lamps, and any desired combinations thereof. In addition to disbanding, surface modifications can also be accomplished by the treatment with gases, plasma and/or liquids, especially if in addition there are reactive substances employed as well. In the present invention, it is preferred that a developing step is performed by thermal treatment and removal of the liquefied portion. The present invention is also directed to a method for generating a relief structure comprising the steps of: a.) the provision of a relief precursor comprising i) a dimensionally stable support, ii) at least one photopolymer layer comprising at least one binder, at least one photoinitiator or photoinitiating system, at least one component with at least one unsaturated group and at least one plasticizer, wherein the melt flow index of a mixture of 30 wt% of the at least one plasticizer with a SIS block copolymer (with a styrene content of 19%, a diblock content of 30%, and a molecular weight Mw 216000 g/mol, a dispersity of 1.04) is higher than 7 g/10 min (MFI 160 °C/1.2 kg), b.) imaging the relief precursor, c.) exposing the imaged relief precursor with electromagnetic radiation to cure the imaged areas, d.) removing of the non-cured areas, and e.) optionally further steps. In step b) of the method for generating a relief structure, the relief precursor is preferably imaged by ablation of a mask layer, by exposure through mask or by direct imaging.

The mask layer can be a separate layer, which is applied to the relief precursor following the removal of a protective layer that may possibly be present, or an integral layer of the precursor, which is in contact with the relief layer or one of the optional layers above the relief layer, and is covered by a protective layer that may possibly be present. The mask layer can also be a commercially available negative, which, for example, can be produced by means of photographic methods based on silver halide chemistry. The mask layer can be a composite layer material in which, by means of image-based exposure, transparent layers are produced in an otherwise non-transparent layer, as described, for example in EP 3 139210 Al, EP 1 735 664 Bl, EP 2987030, Al EP 2313 270 Bl. This can be carried out by ablation of a non transparent layer on a transparent carrier layer, as described, for example, in US 6,916,596, EP 816 920 Bl, or by selective application of a non-transparent layer to a transparent carrier layer, as described in EP 992846 Bl, or written directly onto the relief-forming layer, such as, for example, by printing with a non-transparent ink by means of ink-jet, as described, for example, in EP 1 195 645 Al.

Image wise removal of the mask layer is preferably performed using ablation technology. As a rule, the electromagnetic radiation for ablating the mask will generally be radiation having a wavelength in the range from 300 nm to 20000 nm, preferably in the range from 500 nm to 20000 nm, particularly preferably in the range from 800 nm to 15000 nm, very particularly preferably in the range from 800 nm to 11000 nm. In addition to solid-body lasers, gas lasers or fiber lasers can also be used. Preferably, in laser ablation, use is made of Nd:YAG lasers (1064 nm) or CCh-lasers (9400 nm and 10600 nm). For the selective removal of the mask layer, one or more laser beams are controlled such that the desired printing image is produced.

The direct image exposure can be achieved in that the regions to be cross-linked are exposed selectively. This can be achieved, for example, with one or more laser beams, which are controlled appropriately, by the use of monitors in which specific image points which emit radiation are activated, by using movable LED strips, by means of LED arrays, in which individual LEDs are switched on and off specifically, by means of the use of electronically controllable masks, in which image points, which allow the radiation from a radiation source to pass are switched to transparent, by means of the use of projection systems, in which by means of appropriate orientation of mirrors, image points are exposed to radiation from a radiation source, or combinations thereof. Preferably, the direct exposure is carried out by means of controlled laser beams or projection systems having mirrors. The absorption spectra of the initiators or initiator systems and the emission spectra of the radiation sources must at least partly overlap.

The wavelength of the electromagnetic radiation lies in the range from 200 nm to 20000 nm, preferably in the range from 250 nm to 1100 nm, particularly preferably in the UV range, very particularly preferably in the range from 300 nm to 450 nm. Besides broadband irradiation of the electromagnetic radiation, it can be advantageous to use narrow-band or monochromatic wavelength ranges, such as can be produced by using appropriate filters, lasers or light emitting diodes (LEDs). In these cases, wavelengths of 350 nm, 365 nm, 385 nm, 395 nm,

400 nm, 405 nm, 532 nm, 830 nm, 1064 nm (and about 5 nm to 10 nm below and/or above this), on their own or in combination, are preferred.

In step c) of the method for producing a relief structure, the imaged relief precursor is exposed with electromagnetic radiation to cure the imaged areas. The relief is generated by exposure with electromagnetic radiation through a mask film. On exposure, the exposed regions undergo crosslinking, whereas the unexposed regions of the precursor remain soluble or liquefiable and are removed by appropriate methods. Where an imaged mask is present, irradiation may take place extensively, or, if operating without a mask layer, irradiation may take place in an imaging way over a small area (virtually dotwise) by means of guided laser beams or positionally resolved projection of electromagnetic radiation. The wavelength of the electromagnetic waves irradiated in this case is in the range from 200 to 2000 nm, preferably in the range from 200 to 450 nm, more preferably in the range form 250 nm to 405 nm. The irradiation may take place continuously or in pulsed form or in a plurality of short periods with continuous radiation. In addition to broadband radiation of the electromagnetic waves, it may be advantageous to use narrow-band or monochromatic wavelength ranges, as can be generated using appropriate filters, lasers or light- emitting diodes (LEDs). In these cases, wavelengths in the ranges 350, 365, 385, 395, 400, 405, 532, 830, 1064 nm individually (and about 5-10 nm above and or below) or as combinations are preferred. The intensity of the radiation here may be varied over a wide range, ensuring that a dose is used, which is sufficient to cure the radiation-curable layer sufficiently for the later development procedure. The radiation-induced reaction, possibly after further thermal treatments, must be sufficiently advanced that the exposed regions of the radiation-sensitive layer become at least partially insoluble and therefore cannot be removed in the developing step. The intensity and dose of the radiation are dependent on the reactivity of the formulation and on the duration and efficiency of the developing. The intensity of the radiation is in the range from 1 to 15000 mW/cm ² , preferably in the range from 5 to 5000 mW/cm ² , more preferably in the range from 10 to 1000 mW/cm ² . The dose of the radiation is in a range from 0.3 to 6000 J/cm ² , preferably in a range from 3 to 100 J/cm ² , more preferably in the range from 6 to 20 J/cm ² . Exposure to the energy source may also be carried out in an inert atmosphere, such as in noble gases, C02 and or nitrogen, or under a liquid, which does not damage the relief precursor. Exposure through the mask can be done by using optical devices, for example for beam widening, by a two-dimensional arrangement of multiple point-like or linear sources (for example light guides, emitters), such as fluorescent strip lamps arranged beside one another, by moving a linear source or an elongated arrangement of LEDs (array) relative to the relief precursor, for example by a uniform movement of LEDs or combinations thereof. Preferably, fluorescent strip lamps arranged beside one another or a relative movement between one or more LED strips and the relief precursor is used.

The irradiation can be carried out continuously, in a pulsed manner or in multiple short periods with continuous radiation.

In step d) of the method for producing a relief structure, the non-cured areas are removed. The removal of the non-cured areas of the precursor is preferably performed by treatment with heat and a developing material configured to adsorb non-cured material. More preferably, in step d the precursor is heated to a temperature in the range of 70 to 200 °C, preferably in the range of 80 to 180 °C, more preferably in the range of 90 to 165 °C. The heating of the exposed relief precursor may be carried out by all of the techniques known to the skilled person, such as, for example, by irradiation with IR light, the action of hot gases (e.g., air), using hot rollers, or any desired combinations thereof. To remove the (viscously) liquid regions it is possible to employ all techniques and processes familiar to the skilled person, such as, for example, blowing, suction, dabbing, blasting (with particles and/or droplets), stripping, wiping, transfer to a developing medium, and any desired combinations thereof. Preferably the liquid material is taken up (absorbed and or adsorbed) by a developing medium, which is contacted continuously with the heated surface of the relief precursor. The procedure is repeated until the desired relief height is reached. Developing media, which can be utilized are papers, woven and nonwoven fabrics, and films, which are able to take up the liquefied material and may consist of natural fibers and or polymeric fibers. Preference is given to using nonwovens or non- woven fiber webs of polymers such as celluloses, cotton, polyesters, polyamides, polyurethanes, and any desired combinations thereof, which are stable at the temperatures employed when developing.

Alternatively, in step d) the precursor is treated with a developing liquid to dissolve non- cured material. The techniques applied in this development step may be all of those familiar to the skilled person. The solvents or mixtures thereof, the aqueous solutions, and the aqueous-organic solvent mixtures may comprise auxiliaries, which stabilize the formulation and/or increase the solubility of the components of the non-crosslinked regions. Examples of such auxiliaries are emulsifiers, surfactants, salts, acids, bases, stabilizers, corrosion inhibitors, and suitable combi nations thereof. For development with these solutions, it is possible to use all of the techniques known to the skilled person, such as, for example, dipping, washing or spraying with the developing medium, brushing in the presence of developing medium, and suitable combinations thereof. Preference is given to developing with neutral aqueous solutions or water, with removal assisted by means of rotating brushes or a plush web. Another way of influencing the development is to control the temperature of the developing medium and to accelerate the development by raising the temperature, for example. In this step, it is also possible for further layers still present on the radiation-sensitive layer to be removed, if these layers can be detached during development and sufficiently dissolved and/or dispersed in the developer medium.

In step e) of the method for producing a relief structure, optionally one or more steps of post treatment, post exposure, and/or detackifying are performed. These include, for example, a thermal treatment, drying, a treatment with electromagnetic rays, with plasma, with gases or with liquids, attachment of identification features, cutting to format, coating, and any desired combinations thereof. A thermal treatment may be utilized, for example, to initiate and/or to complete reactions, to increase the mechanical and or thermal stability of the relief structure, and to remove volatile constituents. For the thermal treatment, it is possible to use the known techniques, such as heating using heated gases or liquids, IR radiation, and any desired combinations thereof, for example. In these contexts, it is possible to employ ovens, blowers, lamps, and any desired combinations thereof. In addition to disbanding, surface modifications can also be accomplished by the treatment with gases, plasma and or liquids, especially if in addition there are reactive substances employed as well. Treatment with electromagnetic radiation may be used, for example, for the purpose of detackifying the surfaces of the relief structure, and triggering and or completing polymerization reactions and/or crosslinking reactions. The wavelength of the irradiated electromagnetic waves in this case is in the range from 200 to 2000 nm.

In one embodiment the device capable of heating and mixing the components and forming a melt is selected from the group consisting of a kneader, a (Buss) co-kneader, a single screw extruder, a co- or counter-rotating twin-screw extruder and a multi screw extruder. The selection of suitable extruder screws, the geometries of which have to be matched to the expected processing functions, e.g. intake, conveying, homogenizing, melting, and compressing, is within the general knowledge of the person skilled in the art.

In some embodiments the extruder has a length to diameter ratio in the range of 20 to 150, preferably 40 to 110, more preferably 42 to 60. In general, the extruder comprises at least a transport element, and optional other elements selected from the group consisting of a mixing element, a kneading element, a back pumping element, a barrier element, a degassing element, a cooling element and any combination thereof. Some elements may also combine different functions e.g. degassing and mixing. The position of the different elements along the extruder is governed for example by the temperature profile or the feeding sequence or combinations thereof. Preferably the following setup is used:

The feeding of the components can be carried out in a way that a pre-mixture of all components is fed or a pre-mixture of some of the components is fed first, followed by the other components.

Feeding of the components may be performed in different segments of the device depending on the nature of the components and the sequence of addition. The position of the feeding elements may also depend on the temperature, which is reached at certain positions of the device. Liquid as well as solids may be fed using the proper feeding units.

It is advantageous to eliminate gases formed from volatile components at least partially.

The starting materials are mixed and heated in order to form a homogeneous melt and the temperature is raised from room temperature to temperatures where the polymers start to soften and/or to flow. The temperatures of the different zones of the extruder cylinder are selected to be in the range of 0 °C to 270 °C, preferably in the range of 10 °C to 260 °C, more preferably in the range of 15 °C to 250 °C most preferably in the range of 15 °C to 230 °C. Typically the feeding section of the extruder is cooled to 10 to 25 °C and the temperature is gradually increased to the extmder temperature of 50 to 270 °C, preferably in the range of 60 °C to 240 °C, more preferably in the range of 70 °C to 200 °C, most preferably in the range of 80 °C to 180 °C over a distance of 2D-20D and kept there until the end of the extmder. Optionally the temperature is gradually decreased at the end of the extmder to lower the melt temperature before the melt exits the extmder. The extmder die temperature is adjusted according to common knowledge to assure smooth extrusion of the melt and is typically in the range of 50 to 200 °C.

When the mixing has reached an end the mixture may be actively cooled in an optional cooling step. This can be advantageous e.g. when the mixture needs to be packed into containers or applied to substrates, which are heat sensitive. Cooling may be performed with any method known to the person skilled in the art. Examples for cooling methods to be used are for example cooling with a cooling element attached to the extmder where water or other cooling liquids may be used to cool the element, cooling rolls, cooling belts or passing the extruded material through a liquid cooling bath e.g. filled with water or other liquid, or spraying a cooling liquid or blowing a gas, preferably air, onto the mixture or combinations thereof. In some cases it may be advisable to perform a shaping step before the cooling step, whereby one or more strands or profiles, films, tapes, plates, tubes, rods, or pellets are formed. For manufacturing of printing plate precursors slit dies are preferred in order to generate layers of the photosensitive material.

The present invention is also directed to a method for the production of a relief precursor comprising the steps of: a.) the provision of ingredients comprising at least one binder, at least one photoinitiator or photoinitiating system, at least one component with at least one unsaturated group and at least one plasticizer wherein the melt flow index of a mixture of 30 wt% of the at least one plasticizer with a SIS block copolymer (with a styrene content of 19%, a diblock content of 30%, and a molecular weight Mw 216000 g/mol, a dispersity of 1.04) is higher 7 g/10 min (MFI 160 °C/1.2 kg), b.) the provision of a mixing unit capable of producing a homogeneous mixture of said ingredients, c.) the mixing of the ingredients, and d.) the application onto a dimensionally stable flexible support.

The present invention is furthermore directed to a method for the production of a relief precursor comprising the steps of: a.) the provision of ingredients comprising at least one binder premixed with at least one plasticizer wherein the melt flow index of a mixture of 30 wt% of the at least one plasticizer with a SIS block copolymer (with a styrene content of 19%, a diblock content of 30%, and a molecular weight Mw 216000 g/mol, a dispersity of 1.04) is higher than 7 g/10 min (MFI 160 °C/1.2 kg), at least one photoinitiator or photoinitiating system and at least one component with at least one unsaturated group, b.) the provision of an extruder capable of producing a homogeneous mixture of said ingredients, and c.) the mixing of the ingredients wherein the temperature at the final feeding section is below 140 °C.

The following, non-limiting examples are provided to illustrate the invention.

Determination of UV transmission:

The UV transmission at 365 nm was measured in n-hexane with a plasticizer concentration of 5 parts by weight in a macro-cuvette 110-QS, 10 mm. The UV/Vis spectrum was recorded on a Varian Cary 50, scan Software version: 02.00, beam mode: Dual Beam. A baseline correction was performed with a blank sample of pure n-hexane and applied with the integrated software of the instrument. Determination of the light transmission:

The pure oil was filled in a macro-cuvette 110-QS, 10 mm and the light transmission measured by putting the cuvette in a densitometer Gretag Macbeth D 200 II (measuring tube: V (l), measuring aperture: 3 mm diameter) and pressing the probe head on the macro-cuvette. The measurement of the transparent MCT-oil, which is resistant towards oxidation due to the lack of double bonds, was set as standard with 100% transmission. The mean value of 3 measurements was determined.

Melt Flow Index (MFI) of rubber plasticizer mixtures:

To obtain a homogeneous mixture of rubber and plasticizer, 233 g rubber were dissolved in 412 g toluene and heated to reflux. 100 g of plasticizer were added and stirred until a homogeneous mixture was obtained. No additional stabilizers were added as the rubber was sufficiently stabilized by the manufacturer. The solution was cast on a 125 pm thick PET foil (temperature 20 °C, 30% relative humidity) and excess solution was removed by a doctor blade with a gap of 3160 pm, obtaining a layer of 1200 - 1300 pm rubber with 30 wt% plasticizer content after drying overnight (14 h) at 20 °C (30% relative humidity) and afterwards at 65 °C for 4 h. The rubber-plasticizer layer was peeled off, cut into stripes to charge the melt flow cylinder with at least 6 g of the material and the melt flow index was determined according to ISO 1133-1. The test conditions (temperature of measurement, used weight) are given in brackets for the respective measurement. The MFI increase compared to mineral oil was calculated by dividing the MFI of the respective plasticizer rubber mixture through the MFI of the mineral oil (CAS 8042-47-5, kinematic viscosity at 40 °C of 70 mm ² /s) rubber mixture, and giving the result in percent.

Characterization of the rubber

The styrene content and a diblock content were provided by the technical data sheet of the supplier. The molecular weight was determined by gel permeation chromatography (GPC) (Agilent 1260 infinity pump G1310B, auto-sampler G1329B, Rl-detector G1362A, PSS inline degasser, PSS TCC6000 oven) with a PSS SDV 5 pm 8x50 mm pre-column, PSS SDV 5 pm 1000 A 8x300 mm main column and PSS SDV 5 pm 100.000 A 8x300 mm main column. The rubber sample was dissolved in THF with a concentration of 1.00 g/1. The injection volume was 30 pF and the analysis was run with a flow rate of 1.00 ml/min at a temperature of 25 °C on the columns and 35 °C at the detector. The signal was detected with a RI detector. The calibration was performed with PSS KIT Poly (styrene), Mp 682, 1250, 3250, 8400, 17600, 34800, 66000, 130000, 277000, 549000, 549000, 1210000 Da with a polynomial fit of the 4 ^th order. Table 1: Properties of various plasticizers and MFI with SIS rubber

'Not measured, solid at room temperature. ² Not available, ³ Not melting at 160 °C. Table 2: Properties of various plasticizers and MFI with SBS rubber

'Not measured, solid at room temperature. ² Not available. Example 1

Production of materials in plate form:

A photopolymeric mixture containing

- an SIS triblock block copolymer with a styrene content of 14 to 15%, a diblock fraction of around 26% and a vinyl group content of 7-8% in parts by weight as indicated in table 3;

- 5 parts by weight of hexanediol diacrylate,

- 2.5 parts by weight of benzil dimethyl ketal as photoinitiator, plasticizers in parts by weight as indicated in table 3 and

- 1.5 parts by weight of further constituents such as inhibitors and dyes, was molten at elevated temperatures (120 to 180 °C) in an extruder and calendared via a slot die between a cover film with laser-ablatable mask layer having a thickness of 105 pm and a carrier film having a thickness of 125 pm, thus giving the relief precursor (photopolymer + films) with a total thickness of 4040 pm. The relief precursor was exposed from the backside through the carrier foil with UVA light for 100 s (machine type: Combi Fill, UV output 16 mW/cm ² ). The laser ablatable mask on the front side was imaged with a Xeikon TfxX 20 laser (rotation 8.5 U/s, power 35 W (100%)). The plate was irradiated with UVA light from the front side for 15 minutes (machine type: Combi Fill, UV output 16 mW/cm ² ).

Thermal development: Non-polymerized material and residual black mask layer were removed by thermal development using an Xpress thermal developer (Flint Group). 10 passes at a speed of 0.7 inch/s were used, whereby the temperature was set to 325 °F (163 °C), the IR intensity was set to 40%, the blower intensity was set to 25%, the developer roll speed was set to 100%. During the first 4 passes the pressure was set to 60 psi, followed by 5 passes at 80 psi and one final pass at 40 psi.

Solvent development:

For solvent development, a flowline Fill system with nylosolv A as solvent and a solid content of 4.8 - 5.1%, a brush height setting of 1.5 mm and a solvent temperature of 35 °C was used. Afterwards, the plates were dried at 60 °C for 2 h.

Post exposure:

For detackifying, the developed plates were simultaneously exposed to UVA (11 mW/cm ² ) and UVC light (13 mW/cm ² ) for 10 minutes using a Combi Fill machine (Flint Group).

Hardness measurements: The micro-Shore A hardness was measured on the exposed are, which forms the relief, after exposure, development, optionally drying and post-exposure, using a digi test II-M Shore A instrument (Bareiss Priifgeratebau GmbH), which was installed in the B509 test bed (Bareiss Priifgeratebau GmbH) and was controlled by the DTAA control unit (Bareiss Priifgeratebau GmbH). The measuring head (penetration body with 35° angle) was applied to a solid area for the purpose of the measurement, and was pressed by the digi test II analysis instrument with a pressing force of 235 mN and the hardness value was read off after 3 s. Measurement was carried out twice, and the arithmetic mean was formed. The measurements were carried out on the basis of DIN ISO 7619. The Shore A hardness was determined using a hardness-measure apparatus (type U72/80E, Heinrich Bareiss Priifgeratebau GmbH) according to DIN 53 505.

Melt flow index (MFI) of photosensitive compositions in plate form

The photopolymerizable layer was peeled off the carrier and cover foil, cut into stripes to charge the melt flow cylinder with at least 6 g of the material and the melt flow index was determined according to ISO 1133-1. The test conditions (temperature of measurement, used weight) are given in brackets for the respective measurement.

Torque:

The torque was determined as percentage of the maximal capacity of the extrusion line. The extmsion temperature and processing parameters were kept constant.

Table 3: Results of the properties measurements of various compositions of materials in plate form

'The MFI was determined for the photopolymerizable mixture of all components of example 1.

From the results presented in table 3, it can be concluded that, independent from the type of plasticizer (rapeseed oil/mineral oil), the hardness mostly depends on the content of plasticizer by weight. At the same time, the melt flow rate of the raw plate rubber formulation increases significantly for rapeseed oil compared to the standard plasticizer mineral oil. Hence melt flow rate can be improved using bio-based plasticizers without influencing the hardness.

Example 2: Production of materials in plate form:

A photopolymeric mixture containing:

- 58.0 parts by weight of a SBS triblock copolymer with a styrene content of 30%, no diblock as binder,

- 7.5 parts by weight of hexanediol diacrylate, - 2.0 parts by weight of benzil dimethyl ketal as photoinitiator,

- 31 parts by weight plasticizer and

- 1.5 parts by weight of further constituents such as inhibitors and dyes was melted at elevated temperatures (120 to 180 °C) in an extruder and calendared via a slot die between a cover film with laser-ablatable mask layer having a thickness of 105 pm and a carrier film having a thickness of 175 pm, thus giving the relief precursor (photopolymer + films) with a total thickness of 1240 pm.

Solvent development:

Solvent development was performed as in example 1 with a brush height setting of 0 mm. Table 4: Results of the properties measurements of various compositions of materials in plate form

'The MFI was determined for the photopolymerizable mixture of all components of example 2. From the results presented in table 4, it can be concluded that, independent from the type of plasticizer (rapeseed oil/poly butadiene), the hardness mostly depends on the content of plasticizer by weight. At the same time, the melt flow rate of the raw plate rubber formulation increases and the torque decreases significantly for rapeseed oil. Also for HO-sunflower oil, a decrease in torque compared to the raw plate formulation with polybutadiene was observed.

Properties of the rubbers:

The SIS rubber is a styrene-isoprene-styrene rubber with a styrene content of 19%, a diblock content of 30% and a molecular weight of the triblock fraction of M _w =216000 g/mol, dispersity 1.04. The molecular weight of the diblock fraction is half of the molecular weight of the triblock.

The SBS rubber is a styrene-butadiene-styrene rubber with a styrene content of 26.9 - 29.9% and a diblock content of 15% and a molecular weight of the triblock fraction of M _w =104000 g/mol, dispersity 1.02. The molecular weight of the diblock fraction is half of the molecular weight of the triblock.

Properties of the plasticizers:

The mineral oil (CAS 8042-47-5) used has a kinematic viscosity at 40 °C 70 mm ² /s.

The liquid styrene butadiene rubber Kuraray SBR 820 has a molecular weight of M _w -8500 g/mol.

The liquid isoprene rubber Kuraray LIR-30 has a molecular weight of M _w -28000 g/mol. The poly butadiene used has a molecular weight of M _w < 3000 g/mol and 15 - 25%

1,2-vinyl groups.

The rapeseed oil has a density at 20 °C of 0.916 - 0.923 g/cm ³ , a refractive index at 20 °C of 1.470 - 1.474, an iodine value (g iodine/100 g) of 105 - 126, an acid value (mg KOH/g) below 0.5, and a saponification value (mg KOH/g) of 180 - 195.

The rapeseed oil aged was obtained by heating the same batch of rapeseed oil as used for the other experiments to 160 °C for 19 h.

The sunflower oil has a density at 20 °C of 0.919 - 0.925 g/cm ³ , a refractive index at 20 °C of 1.473 - 1.476, an iodine value (g iodine/100 g) of 120 - 140, an acid value (mg KOH/g) below 0.5, and a saponification value (mg KOH/g) of 184 - 194.

The soybean oil has a density at 20 °C of 0.916 - 0.922 g/cm ³ , a refractive index at 20 °C of 1.465 - 1.475, an iodine value (g iodine/100 g) of 120 - 141, an acid value (mg KOH/g) below 0.5, and a saponification value (mg KOH/g) of 180 - 200.

The palm oil has a melting point of 33 - 42 °C, a refractive index at 40 °C of 1.450 - 1.460, an iodine value (g iodine/100 g) of 50 - 57, an acid value (mg KOH/g) below 0.4, and a saponification value (mg KOH/g) of 190 - 210.

The palm kernel oil has a melting point of 25 - 30 °C, a refractive index at 40 °C of 1.448 - 1.452, an iodine value (g iodine/100 g) of 13 - 23, an acid value (mg KOH/g) below 0.4, and a saponification value (mg KOH/g) of 230 - 254.

The coconut oil has a melting point of 20 - 28 °C, a refractive index at 40 °C of 1.448 - 1.451, an iodine value (g iodine/100 g) of 7 - 12, and an acid value (mg KOH/g) below 0.4.

The MCT oil (medium chain triglycerides 60/40) has a density at 20 °C of 0.930 - 0.960 g/cm ³ , a refractive index at 20 °C of 1.440 - 1.452, an iodine value (g iodine/100 g) of 0 - 1 and an acid value (mg KOH/g) below 0.2, and a saponification value (mg KOH/g) of 325 - 345.

The linseed oil has a density at 20 °C of 0.924 - 0.931 g/cm ³ , a refractive index at 20 °C of 1.478 - 1.483, an iodine value (g iodine/100 g) of 170 - 203 and an acid value (mg KOH/g) below 0.5, a saponification value (mg KOH/g) of 186 - 194, and a Gardner color below 6.0. The castor oil has a density at 20 °C of 0.955 - 0.968 g/cm ³ , a refractive index at 20 °C of 1.478 - 1.480, an acid value (mg KOH/g) below 2, and a Gardner color below 4.0. The HO (high oleic) sunflower oil has a density at 20 °C of 0.912 - 0.920 g/cm ³ , a refractive index at 20 °C of 1.464 - 1.474, an iodine value (g iodine/100 g) of 78 - 90, an acid value (mg KOH/g) below 0.4, and a saponification value (mg KOH/g) of 187 - 197.

In the above, the invention has been disclosed using examples thereof. However, the skilled person will understand that the invention is not limited to these examples and that many more examples are possible without departing from the scope of the present invention, which is defined by the appended claims and equivalents thereof.